There are two variables that determine the number of population growth you need to get one population point. You find both in campaign_variables.
For the first population point the needed growth is: 'number of province regions' * province_development_threshold_cal_points_addition
For each additional point a fixed value is added according to this formula: 'number of provice regions' * province_development_threshold_calc_points_multiplier
The vanilla values for these variables are 3 and 2. Therefore a four region province needs 4 * 3 = 12 growth for the first population point. For each additional point 4*2 is added, so the 2nd point needs 20 growth, the third 28, and so on. With these two variables you can determine the speed of settlement growth without having to change all growth values for buildings, agents, technologies and the like, which may be handy for mods with several turns per year that want to slow down the game progression. Keep in mind that these variables are used for other campaigns like CiG too.